1. Field of the Invention
The invention pertains to a method and a device with a permanent magnet, used to set the static convergence and/or purity of a color television tube.
2. Description of the Prior Art
A color television (or display) tube usually has three electron guns designed to excite luminophors placed against the inner surface of the envelope or front part of the tube. These luminophors are arranged in groups of three dots (triads) or three bands and, in each group, one dot or one band is intended to emit, when excited, a light of a specified color, generally red, green or blue.
In a standard tube, a perforated mask is set before the screen in such a way that each luminophor is excited only by the electron gun designed to excite the corresponding color.
The scanning of the screen by electron beams is obtained through two deflectors, one providing the horizontal deflection and the other the vertical deflection. Each deflector consists of a coil supplied with current. The two coils are generally mounted on one and the same support around the neck and flared part of the television tube.
To obtain a faithful reproduction of the outlines of the images (without colored fringes), all three electron beams must converge to form one and the same virtual light spot on the screen, it being virtual because it is hidden by the mask. This result is obtained by a so-called convergence setting when the tube is being manufactured.
The three electron beams must also have precise positions with respect to the screen, and a gun assigned to one color should reach only those luminophors that produce this color. If not, the colors will not be pure. For this reason, when manufacturing the tube, a so-called purity setting is made.
Settings in which scanning (i.e. the action of the deflectors) is used are called dynamic settings. They consist in the precise positioning of the deflector with respect to the rest of the tube.
Settings in which scanning is not used are called static settings. Purity settings and static convergence settings are of this type. They are done either by moving magnets or by modulating the induction of the poles of a magnetizable ring placed around the neck of the cathode tube The French patent No. 83 06832 which corresponds to U.S. Pat. No. 4,641,062, filed on behalf of the Applicant, describes a method for setting the purity and static convergence of the tube. In this method, a magnetized ring is placed around the neck of the tube and eight poles, evenly distributed around the axis, are created in the said magnetized ring by means of magnetizing coils applied against the external surface of the ring so that a current of a given intensity in a coil engenders a magnetic pole having a clearly defined induction value. The magnetization method is, for example, the one described in the French patent No. 83 06833 which corresponds to U.S. Pat. No. 4,636,694, also filed on behalf of the Applicant, wherein the magnetic material is strongly magnetized and then the magnetization is reduced by reversing it to reach the induction which makes it possible to set the static convergence and/or purity.
It has been observed that in the method in which the end of a coil is applied to the external surface of a magnetizable ring, the setting precision obtained is not satisfactory. It has been discovered that this lack of precision has the following cause: each coil produces, outside its turns, a field with a direction opposite to the one created inside the turns. The useful field is the one produced inside the turns. By contrast, the field with an opposite direction creates, around each pole, a pole with an opposite direction which is the cause of the lack of precision observed. This lack of precision increases with the number of poles.
The invention removes these disadvantages.